1. Field of the Invention
The present invention relates generally to optical transmission lines, and more particularly to advantageous aspects of a dispersion-managed optical transmission line and methods for making the dispersion-managed optical transmission line.
2. Description of the Prior Art
In the area of terrestrial networks, there is increasing interest in all optical transmission over distances many times greater than the traditional 400-500 km spacing between centers of electronic regeneration and signal switching. This interest is driven, in part, by the economics of dense wavelength division multiplexing (WDM), which allows multiple signals of varying wavelength to be carried by a single optical line.
As all-optical transmission lengths increase, it is increasingly important to manage the dispersion of optical signals along the line. There is a considerable demand for providing a length of fiber optic cable having a specified amount of path-average dispersion {overscore (D)}. The path-average dispersion {overscore (D)} for a given span of optical fiber of length L may be expressed mathematically as follows:       D    _    =            1      L        ⁢                  ∫        0        L            ⁢                        D          ⁢                      (            x            )                          ⁢                  xe2x80x83                ⁢                  ⅆ          x                    
where the lead end of the optical fiber is mapped to the origin, and where D(x) is the dispersion parameter D at each point x along the span of optical fiber away from the lead end.
It is useful to express {overscore (D)} in the above manner because although a particular reel of optical fiber may be known to have a certain dispersion, it does not necessarily follow that a section of optical fiber cut from that reel will have the same dispersion. The reason that this is so is illustrated in FIG. 1, which shows a graph 1 of the function D(x) for a length of positive-dispersion optical fiber. The graph in FIG. 1 was generated using a dispersion optical time-domain reflectometer (OTDR). As shown in FIG. 1, the value for the dispersion parameter D varies significantly and irregularly along the length of the fiber. The dispersion function is irregular because of inhomogeneities in optical fiber introduced by the manufacturing process. Thus, depending upon where the fiber is cut, {overscore (D)} for the cut section of fiber may be higher or lower than the overall {overscore (D)} for the entire reel of fiber.
In order to calculate the value of {overscore (D)} for a given span of fiber, the function D(x) is integrated to determine the area underneath D(x) between the endpoints of the span. As expressed in equation (1), above, that area is then divided by the length of the fiber span to determine the average value for D along the length of the span. FIG. 2 shows a graph 2 of the integral ∫D(x) dx of the dispersion function D(x) shown in FIG. 1.
For applications involving the transmission of non-return-to-zero (NRZ) data, the desired overall value for {overscore (D)} for the transmission line is zero. For soliton data transmission, the desired {overscore (D)} is in the range of approximately 0.05 to 0.5 picoseconds per nanometer per kilometer (ps/nm-km). One way of achieving the desired {overscore (D)} is to couple a length of optical fiber having a positive dispersion characteristic with a length of optical fiber that has been doped to have a negative dispersion characteristic. The desired path-average dispersion {overscore (D)} of the transmission line is obtained by adjusting the relative lengths of the positive-dispersion and negative-dispersion fibers. However, because of the above-described irregularities in the dispersion characteristics of optical fiber, the process of piecing together spans of positive-dispersion and negative dispersion fiber to achieve a precise desired overall path-average dispersion {overscore (D)} has proven to be challenging.
The original process used to construct a transmission line from positive-dispersion and negative-dispersion fibers to achieve a desired path-average dispersion {overscore (D)} was based upon trial and error. Because of the inhomogeneities in the fiber, this was a frustrating and time-consuming process, and wasted significant amounts of expensive optical fiber. These and other issues in the art are addressed by U.S. Pat. No. 6,011,615, entitled xe2x80x9cFiber Optic Cable Having a Specified Path Average Dispersion,xe2x80x9d assigned to the assignee of the present application, which is hereby incorporated by reference in its entirety. This patent describes a technique for precisely determining the respective lengths of the two fibers based upon a graphical analysis of the fibers"" dispersion maps.
However, a further issue has arisen in the construction of an ultra-long transmission line for use in dense WDM. Dense WDM employs a relatively wide range of wavelengths, i.e., 1530-1565 mm and well beyond. In order to be suitable for dense WDM, the path-average dispersion {overscore (D)} for a given length of transmission line must be substantially uniform for the full range of signal wavelengths used. In other words, the slope       ∂    D        ∂    λ  
of the dispersion of the transmission line relative to signal wavelength must be zero.
Thus, in a two-fiber WDM transmission line, the compensating, negative-dispersion fiber must not only compensate for the signal dispersion introduced by the positive-dispersion fiber to achieve the desired path-average dispersion {overscore (D)}, but must also compensate for the slope       ∂    D        ∂    λ  
of the positive-dispersion fiber, such that the slope       ∂    D        ∂    λ  
of the two combined fibers is zero. However, depending upon the requirements of the transmission line, it may be necessary or desirable to use a positive-dispersion fiber in conjunction with a negative-dispersion fiber that does not precisely compensate for both dispersion and       ∂    D        ∂    λ  
at the same time. Further, even where the negative-dispersion fiber used in the transmission line compensates for both dispersion and             ∂      D              ∂      λ        ,
the average       ∂    D        ∂    λ  
may nonetheless require adjustment.
There is thus a need for a technique for adjusting the average       ∂    D        ∂    λ  
of a given length of transmission line, while maintaining a desired overall path-average dispersion {overscore (D)}. Further, such a technique must take into account the above-discussed inhomogeneities in the optical fibers making up the transmission line.
The above-described issues and others are addressed by the present invention, one aspect of which provides a method for constructing an optical fiber transmission line having a desired length and path-average dispersion {overscore (D)}, while also having a desired total             ∂      D              ∂      λ        .
The method comprises selecting first and second fibers having dispersions of opposite sign. A third fiber is then selected, having dispersion and       ∂    D        ∂    λ  
characteristics such that when a transmission line having a desired total length and path-average dispersion {overscore (D)} is assembled from the first, second, and third fibers, the total       ∂    D        ∂    λ  
of the transmission line may be adjusted by adjusting the respective lengths of the first, second, and third fibers, while maintaining the desired total length and path-average dispersion {overscore (D)} of the transmission line. According to a further aspect of the invention, the respective lengths of the first, second, and third fibers are calculated using the respective dispersion maps of the three fibers.
Additional features and advantages of the present invention will become apparent by reference to the following detailed description and accompanying drawings.